1. Field of the Invention
This invention relates generally to a method and apparatus for quick determination of the ellipticity of an earth borehole using statistical analysis of distance measurements provided by acoustic sensors.
2. Description of the Related Art
The ellipticity of a borehole traversing an earth formation is useful in ascertaining other valuable information regarding various properties of the formation, such as stresses, porosity, and density. Additionally, borehole ellipticity is useful in evaluating well bore stability and hole cleaning operations. Several methods to obtain information about the ellipticity of a borehole are described in U.S. Pat. No. 5,469,736 to Moake, U.S. Pat. No. 5,638,337 to Priest, U.S. Pat. No. 5,737,277 to Priest, and references cited therein, each of which is incorporated herein by reference. Such methods generally employed acoustic or mechanical calipers to measure the distance from the tool to the borehole wall at a plurality of points around the perimeter of the tool. However, those methods have several drawbacks.
For example, various wireline tools having mechanical calipers have been used to mechanically measure the dimensions of a borehole. However, those techniques require the removal of the drillstring, which results in costly down time. Additionally, such techniques do not allow measurement while drilling (MWD). Moreover, the method described in the '736 patent to Moake appears to be based on the assumption that the borehole shape is circular, or at least that the shape may be approximated by an "equivalent" circle, i.e., a circle having an area equivalent to that of the actual borehole. A significant drawback to that method is that, in reality, the borehole shape is often not circular but is rather of an elliptical shape. Therefore, under many circumstances, that method does not accurately describe the true borehole shape. Furthermore, although the methods described in the '337 and '277 patents do account for the ellipticity of a borehole and tool rotation during measurement, those methods assume that the tool does not translate in the borehole during measurement. During drilling operations, however, the tool is rarely free from translational motion. Thus, those methods generally do not provide satisfactory results in an MWD mode of operation. Another drawback of those methods is that the calculations are too complex and slow for some drilling operations, particularly wiping, sliding, or tripping operations. Moreover, many of those methods require excessive downhole computing power. Thus, there is a need for increased speed and a reduction in the required downhole computing power in determining the ellipticity of the borehole so that the calculations may be made during any drilling operation.
It would, therefore, be a significant advance in the art of petroleum well drilling and logging technology to provide a method and apparatus for quickly and accurately determining the ellipticity of an earth borehole while drilling the borehole or while wiping, sliding, or tripping.